This invention relates firstly to surface light source devices and in particular to such devices of the edge light type. This invention relates additionally to optical elements such as prism sheets which are used in such surface light source devices, as well as to apparatus using such surface light source devices such as image display apparatus, automatic teller machines and game tables.
Liquid crystal (LC) display apparatus, because they have the favorable characteristics of being light and thin, have been used as display devices not only for lap-top and book-type personal computers and word processors but also for electronic notebooks, portable telephones, LC television sets, various portable terminals and video cameras. More recently, they are also being used as display apparatus for measurement instruments such as time counters, overhead display of virtual reality and LC projectors.
Among these LC display apparatus, there are those having a vertically downward-facing surface light source device disposed on the back surface of a LC display panel (hereinafter referred to as the LCD panel), as well as those having an edge-light type surface light source device. FIGS. 1A and 1B show a surface light source device 1 of the former kind, having a linear light source 4 such as a cold cathode ray tube (a fluorescent tube) disposed on the back surface of diffusion plates 2 and 3 and a reflector 5 further behind the linear light source 4 such that the emitted light from the linear light source 4 can be diffused by the diffusion plates 2 and 3 and uniformly projected out from the projecting surface. Because a plurality of linear light sources can be disposed behind the diffusion plates, an LC display apparatus using such a vertically downward-facing surface light source device can provide a high degree of brightness. For obtaining a uniform brightness over the entire light-emitting surface, however, a certain distance must be maintained between the light source and the diffusion plates, causing the overall thickness of the surface light source device to increase. This makes it difficult to produce thin LC display apparatus.
Edge-light type surface light source devices have the advantage that the light source can be made thin because the linear light source is positioned at a side of a light conducting plate. Because of this advantage, more and more apparatus are coming to use edge-light type surface light source devices, as the demand to reduce the thickness of LC display apparatus is becoming greater.
FIG. 2 shows an edge-light type surface light source device 6, with a portion removed, including optical elements such as a linear light source 7, a reflector 8, a light conducting plate 9, a light-reflecting plate 10, a diffusion plate 11 and a pair of converging lens plates 12 and 13. The linear light source 7 and the reflector 8 are disposed by a (light-incident) side surface of the optically transparent light conducting plate 9 such that the light emitted from the linear light source 7 enters the light conducting plate 9 through this side surface either directly or after being reflected by the reflector 8. Side-surface reflecting plates (shown at 14 in FIG. 5) of a metallic dielectric material with a rough surface are provided on side surfaces of the light conducting plate 9 other than the light-incident surface. A cold cathode ray tube (fluorescent tube) is shown as the linear light source 7. A straight single tube or an L-shaped tube may be used, depending on the brightness of display required of the LC display apparatus 6.
A diffusion layer 15 is formed on the lower surface of the light conducting plate 9, and the light-reflecting plate 10 is disposed therebelow. The diffusion layer 15 may be produced by depositing dots of light-diffusing paint or the like by a screen-printing method such that the area of the diffusion layer 15 increases gradually as the distance from the linear light source 7 increases, as shown by examples in FIGS. 3A and 4B.
The efficiency, by which light from the linear light source 7 can be led to the upper surface, will be discussed next. Assume now that the diffusion layer 15 did not exist on the lower surface of the light conducting plate 9. Light beam F1 shown in FIG. 5 indicates a beam which made incidence onto the light incident side surface 16 of the light conducting plate 9 with an angle of incidence 90 degrees from its normal line, that is, its angle of refraction .theta..sub.1 equals the critical angle for the total reflection inside the light conducting plate 9. If the index of refraction for air is n.sub.1 and that of the light conducting plate 9 is n.sub.2, it is known that .theta..sub.1 =sin.sup.-1 (n.sub.1 /n.sub.2), and the angle of incidence .theta..sub.2 of the beam F1 at the lower surface of the light conducting plate 9 is given by .theta..sub.2 =90 degrees-.theta..sub.1. If the light conducting plate is of polycarbonate, n.sub.2 =1.59 and hence .theta..sub.1 =38.97 degrees and .theta..sub.2 =51.03 degrees. Since this angle of incidence .theta..sub.2 is greater than the critical angle .theta..sub.1 for total reflection, light beam F1 will undergo total reflection at the lower surface of the light conducting plate 9 if the diffusion layer 15 is not present on the lower surface of the light conducting plate 9. Similarly, total reflection will take place also at the upper surface of the light conducting plate 9.
Consider another light beam F2 entering from the linear light source 7 into the light conducting plate 9. Since its angle of refraction .theta..sub.3 is smaller than .theta..sub.1, its angle of incidence E4 at the upper and lower surfaces of the light conducting plate 9 is larger than .theta..sub.2. Accordingly, light beam F2 from the linear light source 7 undergoes total reflections at both upper and lower surfaces of the light conducting plate 9 if there is no diffusion layer 15.
Since the reflecting plates 14 are provided on the other side surfaces of the light conducting plate 9 (that is, other than the light incident side surface 16), light which is reflected on them is nearly entirely reflected back into the interior of the light conducting plate 9. Since the angle of incidence at the upper and lower surfaces does not change by such reflections, light beam F2 continues to undergo total reflection. Loss of light may be considered negligible by reflection by the reflecting plates 14 made of a metallic dielectric material.
Next, consider the light source. If a cold cathode ray tube is used as the linear light source 7, the surface of the glass tube of such a cold cathode ray tube is covered with a fluorescent layer having a property of total diffusion against light from outside. In other words, light which is made incident onto the linear light source 7 is reflected therefrom without any loss.
Thus, the light conducting plate 9 without the diffusion layer 15 on its lower surface can seal in with a very high efficiency any light which enters from the linear light source 7, but a plate which seals in incident light cannot serve as a light source. The sealed light must be allowed to come out through a light emitting surface 17 (the upper surface of the light conducting plate 9). This is why the diffusion layer 15 is provided on the lower surface of the light conducting plate 9 such that light which is incident on the diffusion layer 15 is diffused and that portion of the light which does not satisfy the condition for total reflection is allowed to escape. This escaped portion of light is further diffused by the diffusion plate 11 on the upper surface of the light conducting plate 9.
In summary, light from the linear light source 7 is emitted with a very high efficiency towards the display surface of the LC display apparatus. Even light coming from the display surface is similarly re-emitted towards the display surface without any loss.
Diffusion of light from such an edge-light type surface light source device 6 is illustrated in FIG. 6. Light beam F3 reflected on the lower surface is diffused as Lambert beam, and the portion which does not satisfy the condition for total reflection is emitted out through the upper surface of the light conducting plate 9 as a semi-spherical beam F4. Light beam F4 is further diffused by the diffusion plate 11, becoming Lambert beam F5. This passes through the two converging lens plates 12 and 13 and is emitted upwards as beam F6.
When such a surface light source is used as a back-light source for an LC display apparatus, however, the brightness is not sufficient. In order to increase the front brightness of an LC display apparatus, it is generally required that the direction of light emission from the surface light source device 6 should be unidirectionally aligned. Another reason for low front brightness is the low opening ratio of the LCD panel. As shown generally at 21 in FIG. 7, the LCD panel has liquid crystal 29 sealed between a glass plate 25 having thin-film transistors (TFT) 22, wiring 23 and a black matrix 24 formed on its upper surface and another glass plate 28 having a color filter 26 and a transparent electrode 27 formed on its lower surface and polarization plates 30 and 31 thereabove and therebelow. The areas covered by the black matrix 24 serve to screen the light from the surface light source device 6, and only the open areas 32 surrounded by the black matrix 24 allow the light to pass through. Because the ratio of these openings is low, sufficient brightness cannot be obtained on the display surface of the LC display apparatus. If it is desired to make the image elements (pixels) very small in order to improve the image quality of the LCD panel 21, in particular, the open areas 32 become small because there is a limit to how small the black matrix 24 can be made.
One way to minimize the reduction in brightness due to the black matrix 24, as shown in FIG. 8, is to use a micro-lens array 33 to focus the light emitted from the surface light source device 6 at the open areas 32 of the LCD panel 21 such that all light beams will pass through the openings. If there are fluctuations in the direction of light from the surface light source device 6, however, the micro-lens array 33 cannot focus light at small open areas 32, and the brightness cannot be successfully made higher.
FIG. 9 shows the relationship between the angle of light emission (measured from a line perpendicular to the display surface of an LC display apparatus) and brightness, Curve A indicating the brightness of a pixel portion where the TFT is on and it is in the light-transmitting condition and Curve B indicating the brightness of a pixel portion where the TFT is off and it is in the light-non-transmitting condition. The angle of emission is defined negative on the side of the light source. FIG. 9 shows that the brightness-darkness contrast is great in the frontal directions of the LC display apparatus but the light transmissivity is low and the contrast is poor in diagonal directions. If the display surface is looked at diagonally at a very large angle, the brightness-darkness contrast may be inverted or the displayed color may appear differently.
When an LC display apparatus is used in a device to be looked at by many viewers such as a television set, it is necessary to make the display surface visible also from directions other than the frontal direction. Since LC display apparatus are not easily visible from diagonal directions, it may be considered feasible, as shown by broken line in FIG. 8, to place a diffusion plate 34 on top of the LCD panel 21 such that light emitted from the LCD panel 21 can be caused to propagate also sideways. If such a diffusion plate 34 is installed on the side of the surface of the LCD panel 21, however, light-emitting points come to be on the diffusion plate 34. Thus, if use is made of a surface light source device with fluctuations in the direction of light emission, beams of light which passed through mutually adjacent pixels may overlap each other on the diffusion plate 34, resulting in a poorly focused image.
If a color filter is used in a color LC display apparatus, the brightness of the display surface becomes lower because each pixel allows only light within a specified range of wavelength to pass and the amount of transmitted light becomes at most about one third of the amount of incident light.
FIG. 11 shows an attempt to solve this problem by dispersing the white light from the surface light source device 6 into red (R), green (G) and blue (B) colors by means of a diffraction grating 35 and focusing light of each color by means of a micro-lens array 36. This method can be successful, however, only if the beams of light emitted from the surface light source device 6 is unidirectionally aligned.
The polarization plates 30 and 31, which are disposed above and below, further serve to cut polarized light in one direction. Thus, the amount of transmitted light is further reduced by one half, further reducing the brightness of the display surface.
In view of the above, it has been suggested to make use of a polarization separator plate 37, as shown in FIG. 12, instead of the lower one of the polarization plates. Of the light beams emitted from the surface light source device 6, light beams polarized in a specified direction (referred to as the P-polarized light) can pass through both the separation plate 37 and the upper polarization plate 30, but light beams polarized in the perpendicular direction (referred to as the S-polarized light) are reflected by the separation plate 37 and return to the surface light source device 6. The returned S-polarized light is diffused inside the surface light source device 6 and emitted again as unpolarized light. As this process is repeated, all light emitted from the surface light source device 6 is taken out as P-polarized light from the LCD panel 21. This method, too, requires that the light emitted from the surface light source device 6 be unidirectionally aligned.
In summary, in order to solve the problems of prior art surface light source devices such as low front brightness, lower brightness in diagonal directions, lowering of brightness due to the black matrix used in the LCD panel and lowering of brightness due to a color filter of polarization plates, emitted light must be all in one direction.
In other words, light emitted from the surface light source device must be converged and collimated. As shown in FIG. 6, prior art edge-light type surface light source devices were provided with a pair of converging lens plates 12 and 13 to converge emitted light. With prior art edge-light type surface light source devices, however, light convergence cannot be effected satisfactorily because the light which is emitted in all directions from the light-emitting surface 17 of the light conducting plate 9 is once converted into Lambert beam by the diffusion plate 11 and this is then made convergent by means of the converging lens plates 12 and 13. With prior art edge-light type surface light source devices, furthermore, the diffusion plate and the converging lens plates are stacked on top of the light conducting plate 9 such that a loss of light occurs also through these plates, adversely affecting the overall brightness. Many attempts have been made to improve the brightness of LC display apparatus but none has so far been satisfactory.